The present invention relates to trays, for example, dual-ovenable, heat-sealable, food-packaging trays.
Packaging that can withstand exposure to the heating and/or cooking environment of a selected type of oven is said to be “ovenable” with respect to that type of oven. To be ovenable with respect to a microwave oven, the packaging should not, for example, include materials such as metals that reflect microwaves to cause arcing or otherwise damage the oven's microwave generation. To be ovenable with respect to a conventional oven, the packaging should, for example, be able in use to withstand exposure to 400° F. air temperature for up to four hours. Packaging that is ovenable both with respect to a microwave oven and a conventional oven is said to be “dual-ovenable.”
Food packaging may be formed by heat sealing a thermoplastic film lid to a tray supporting the food. This heat sealing operation typically occurs at the food packager's plant using a heat sealing machine designed for high speed operation. Although there are several variations, generally a heat sealing machine includes a heated seal bar that contacts and compresses the lid with a flange of the tray to heat seal them together.
Three variables are important in forming a heat seal: 1) the seal bar temperature, 2) the dwell time, and 3) the sealing pressure. The seal bar temperature is the surface temperature of the seal bar. The dwell time is the length of time that the heated seal bar contacts the lid film to transfer heat from the seal bar to soften at least a portion of the lid. The sealing pressure is the amount of force that squeezes the lid and tray together during this heat transfer. All of these variables interact in completing a successful heat seal.
Because the heat sealing layers of many thermoplastic packaging films used in food packaging are based on relatively low-melting polyolefin thermoplastics (or similar melt-temperature thermoplastics), the heat sealing machines present in food packaging plants are often designed and set to operate with a seal bar temperature, a dwell time, and a sealing pressure in a range useful for such materials. This permits the heat sealing machines to operate at high speeds to form strong seals.
However, such films with polyolefin-based sealant layers generally would not be able to form heat seals that are ovenable in conventional ovens because the relatively low-melting polyolefin layer would melt, decompose, or delaminate upon exposure to a 400° F. conventional oven for normal cooking conditions of up to four hours.
It would be desirable to be able to form a heat seal between a lid film and a tray at “polyolefin-type” of heat-seal conditions, yet have a heat seal that is dual-ovenable (e.g., would be maintained upon exposure to conventional oven conditions). It would be convenient for consumers to be able to place a packaged food comprising a lid film heat sealed to a tray directly into a microwave or conventional oven to heat or cook the food without first having to remove the packaging. The consumer would thus avoid having to handle the raw product or to clean a container in which the food would have otherwise been placed for cooking or heating. The consumer would also simply be able to dispose of the packaging material after heating or cooking the food.